Reduced sarcoplasmic reticulum content of releasable Ca2+ in rat soleus muscle fibres after eccentric contractions.

AIM The purpose was to evaluate the effects of fatiguing eccentric contractions (EC) on calcium (Ca2+) handling properties in mammalian type I muscles. We hypothesized that EC reduces both endogenous sarcoplasmic reticulum (SR) content of releasable Ca2+ (eSRCa2+) and myofibrillar Ca2+ sensitivity. METHODS Isolated rat soleus muscles performed 30 EC bouts. Single fibres were isolated from the muscle and after mechanical removal of sarcolemma used to measure eSRCa2+, rate of SR Ca2+ loading and myofibrillar Ca2+ sensitivity. RESULTS Following EC maximal force in whole muscle was reduced by 30% and 16/100 Hz force ratio by 33%. The eSRCa2+ in fibres from non-stimulated muscles was 45 +/- 5% of the maximal loading capacity. After EC, eSRCa2+ per fibre CSA decreased by 38% (P = 0.05), and the maximal capacity of SR Ca2+ loading was depressed by 32%. There were no effects of EC on either myofibrillar Ca2+ sensitivity, maximal Ca2+ activated force per cross-sectional area and rate of SR Ca2+ loading, or in SR vesicle Ca2+ uptake and release. CONCLUSIONS We conclude that EC reduces endogenous SR content of releasable Ca2+ but that myofibrillar Ca2+ sensitivity and SR vesicle Ca2+ kinetics remain unchanged. The present data suggest that the long-lasting fatigue induced by EC, which was more pronounced at low frequencies (low frequency fatigue), is caused by reduced Ca2+ release occurring secondary to reduced SR content of releasable Ca2+.